Marine seismic cables include a variety of electronics modules fitted between the sections of cable and connected to these via couplings, usually annular couplings. When a fault arises in such an electronics module, it will be necessary to replace it. The usual practice is to haul in the seismic cable and replace the electronics module on board the towing vessel. This is a time-consuming and very expensive operation, especially in the case of surveys in which each vessel is towing a number of seismic cables. If therefore it is possible for the change of the electronics module to be carried out in situ, i.e. while the cable is in the water, both time and expense will be saved.